1. Field of the Invention
The invention relates to carrier frequency offset detection, and more particularly, to methods and receivers using the hierarchical modulated pilots to detect carrier frequency offset, applied in orthogonal frequency division multiplexing systems.
2. Description of the Related Art
Recently, the application of orthogonal frequency division multiplexing system has become the most important wireless communication technology. Data can be easily and efficiently transmitted and received in the wireless communication environment because of the high transmission rate of the orthogonal frequency division multiplexing system. Therefore, orthogonal frequency division multiplexing scheme is applied to, for example, Digital Audio Broadcasting (DAB), Digital Video Broadcasting-Terrestrial/Handheld (DVB-T/H), Wireless Fidelity (Wi-Fi) and Worldwide Interoperability for Microwave Access (WiMAX) etc., and the orthogonal frequency division multiplexing scheme is regarded as the 4th-Generation Wireless System.
The data is transmitted by a plurality of sub-carriers which overlap and are orthogonal to each other in the orthogonal frequency division multiplexing system. In addition, duplicated data which is copied from the end of the symbol is defined as a cyclic prefix (CP) or a guard interval (GI), and the purpose of the duplicated data is to protect the orthogonal frequency division multiplexing symbol from inter-symbol interference (ISI) generated by multi-path fading and reflection in the channels. The bandwidth used by the orthogonal frequency division multiplexing system is divided into a number of sub-bands, and the sub-bands are only affected by the flat fading. Thus, the receiver only needs one simple equalizer to adjust signal gain and compensate for the flat fading of the channel. The orthogonal frequency division multiplexing system has many advantages such as an advantage against multi-path fading, high-efficiency bandwidth, low-complexity equalizer and high transmission rate.
However, the orthogonal frequency division multiplexing system is affected by the Doppler effect in the high speed movement condition such as high speed rail. The orthogonal frequency division multiplexing system which is regarded as a multi-carrier system is very sensitive to carrier frequency offset (CFO) caused by Doppler effect. The carrier frequency offset will destroy the orthogonality between the sub-carriers and generate inter-carrier interference (ICI) between the sub-carriers such that the performance of the orthogonal frequency division multiplexing system in the environment of the high speed decreases and the bit error rate increases. Therefore, how to detect carrier frequency offset (CFO) to cancel inter-carrier interference (ICI) between the sub-carriers is the most important subject to realize the orthogonal frequency division multiplexing system.